ipv4: Allow ipv6 gateway with ipv4 routes

Add support for RTA_VIA and allow an IPv6 nexthop for v4 routes:
   $ ip ro add 172.16.1.0/24 via inet6 2001:db8::1 dev eth0
   $ ip ro ls
   ...
   172.16.1.0/24 via inet6 2001:db8::1 dev eth0

For convenience and simplicity, userspace can use RTA_VIA to specify
AF_INET or AF_INET6 gateway.

The common fib_nexthop_info dump function compares the gateway address
family to the nh_common family to know if the gateway should be encoded
as RTA_VIA or RTA_GATEWAY.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 57 insertions, 3 deletions

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index f99a2ec32505..310060e67790 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -665,10 +665,55 @@ const struct nla_policy rtm_ipv4_policy[RTA_MAX + 1] = {
 	[RTA_DPORT]		= { .type = NLA_U16 },
 };
 
+int fib_gw_from_via(struct fib_config *cfg, struct nlattr *nla,
+		    struct netlink_ext_ack *extack)
+{
+	struct rtvia *via;
+	int alen;
+
+	if (nla_len(nla) < offsetof(struct rtvia, rtvia_addr)) {
+		NL_SET_ERR_MSG(extack, "Invalid attribute length for RTA_VIA");
+		return -EINVAL;
+	}
+
+	via = nla_data(nla);
+	alen = nla_len(nla) - offsetof(struct rtvia, rtvia_addr);
+
+	switch (via->rtvia_family) {
+	case AF_INET:
+		if (alen != sizeof(__be32)) {
+			NL_SET_ERR_MSG(extack, "Invalid IPv4 address in RTA_VIA");
+			return -EINVAL;
+		}
+		cfg->fc_gw_family = AF_INET;
+		cfg->fc_gw4 = *((__be32 *)via->rtvia_addr);
+		break;
+	case AF_INET6:
+#ifdef CONFIG_IPV6
+		if (alen != sizeof(struct in6_addr)) {
+			NL_SET_ERR_MSG(extack, "Invalid IPv6 address in RTA_VIA");
+			return -EINVAL;
+		}
+		cfg->fc_gw_family = AF_INET6;
+		cfg->fc_gw6 = *((struct in6_addr *)via->rtvia_addr);
+#else
+		NL_SET_ERR_MSG(extack, "IPv6 support not enabled in kernel");
+		return -EINVAL;
+#endif
+		break;
+	default:
+		NL_SET_ERR_MSG(extack, "Unsupported address family in RTA_VIA");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 static int rtm_to_fib_config(struct net *net, struct sk_buff *skb,
 			     struct nlmsghdr *nlh, struct fib_config *cfg,
 			     struct netlink_ext_ack *extack)
 {
+	bool has_gw = false, has_via = false;
 	struct nlattr *attr;
 	int err, remaining;
 	struct rtmsg *rtm;
@@ -709,13 +754,16 @@ static int rtm_to_fib_config(struct net *net, struct sk_buff *skb,
 			cfg->fc_oif = nla_get_u32(attr);
 			break;
 		case RTA_GATEWAY:
+			has_gw = true;
 			cfg->fc_gw_family = AF_INET;
 			cfg->fc_gw4 = nla_get_be32(attr);
 			break;
 		case RTA_VIA:
-			NL_SET_ERR_MSG(extack, "IPv4 does not support RTA_VIA attribute");
-			err = -EINVAL;
-			goto errout;
+			has_via = true;
+			err = fib_gw_from_via(cfg, attr, extack);
+			if (err)
+				goto errout;
+			break;
 		case RTA_PRIORITY:
 			cfg->fc_priority = nla_get_u32(attr);
 			break;
@@ -754,6 +802,12 @@ static int rtm_to_fib_config(struct net *net, struct sk_buff *skb,
 		}
 	}
 
+	if (has_gw && has_via) {
+		NL_SET_ERR_MSG(extack,
+			       "Nexthop configuration can not contain both GATEWAY and VIA");
+		goto errout;
+	}
+
 	return 0;
 errout:
 	return err;